Australia’s 116-year-old constitution states that a “subject or citizen of a foreign power” is not eligible to be elected to parliament.

Mr Joyce was born in Australia but found out in August that he had automatically acquired New Zealand citizenship from his father – and the court could rule that his election in July 2016 was illegal, prompting a by-election.

The remaining six senators could be replaced by members of the same party without the need for another vote.

In the High Court, a government lawyer argued that five of the seven politicians should not be disqualified because they did not voluntarily acquire or retain citizenship of another country.

Image:Malcolm Turnbull could lose his one-seat majority in the House of Representatives

Solicitor-general Stephen Donaghue said: “If a person is not aware either that they are a dual citizen or of a significant prospect that they are, in our submission by definition that person cannot have a split allegiance.”

Mr Donaghue argued that only Scott Ludlam, born in New Zealand, and Malcolm Roberts, born in India, should banned for not renouncing their dual nationalities. Mr Ludlam has since resigned.

Support for Prime Minister Turnbull is at a six-month low, but political analysts believe he might receive a boost if he is able to win passage of a same-sex marriage bill.

The dual citizenship rule was originally inserted into the 1901 constitution to ensure parliamentarians were loyal solely to Australia.

However, critics say it is out of step with the country’s modern reality, where 50% of the population are either foreign born or the children of immigrants.